At present, about 14,000,000 tons of zinc ingots are consumed in the world annually, wherein 50% of the zinc ingots are used for surface protection of steel. In the recovery of galvanized steels, zinc is volatilized at high temperature and incorporated into steelmaking dust, and this kind of dust is called electric arc furnace dust (EAFD). In addition, zinc is also associated with various metal mineral resources, and the zinc is also incorporated into the smelting dust during a process of smelting those metals at high temperature. In some zinc-containing wastes, lead and zinc are volatilized and incorporated into the dust through a high-temperature reduction volatilization. These kinds of dust are the main source of secondary zinc resources. These zinc-containing materials are common in that zinc exists in the form of oxide. Meanwhile, these zinc-containing materials are different in terms of chlorine content, which exceeds the maximum amount allowed by conventional zinc hydrometallurgy by tens folds to hundreds folds, and these zinc-containing materials contain lead, calcium, magnesium, iron, sodium, potassium, silicon dioxide, etc. When being treated by current zinc hydrometallurgy technology, chlorine enters into the solution which prevents the electrolysis process of the zinc from occurring. Mineral zinc raw materials are growing increasingly tense and running out around the world, while the amount of secondary zinc resources has been increasing, and accumulation of the secondary zinc resources has caused increasing pressure on the environment. Therefore, people are urged to conduct a lot of researches on utilization of the secondary resources.
High chlorine content is a key factor as to why the current zinc smelting technology is unavailable to the treatment of the secondary resources. In order to adapt the raw material to the technological requirements of zinc hydrometallurgy, current researches mainly focus on removal of chlorine in the raw materials. According to a concept of the existing treatment, chlorine of the materials is removed, and the current zinc smelting technology is employed after reducing the chlorine content.
Chlorine is removed from the chlorine-containing zinc oxide through pyrogenic dechlorination and wet dechlorination processes, respectively. The pyrogenic dechlorination relies on the basis that metal chloride has a higher vapor pressure at high temperature and has a property of being easily volatilized, hence chlorine is removed and volatilized in the form of metal chloride at high temperature. Generally, such dechlorinating equipment includes multi-hearth furnace and waelz kiln. Some people have also studied the use of microwave in dechlorination, which heats the materials to 700-1100° C. to volatilize the chlorine. The pyrogenic dechlorination has shortcomings such as high energy consumption, low metal recovery rate, creation of a gas-phase pollution source, and imposing pressure on the environment due to generation of dust with even higher chlorine content. However, the pyrogenic dechlorination is still widely adopted by various enterprises at present. After removing the chlorine, the zinc oxide is applied to the existing zinc smelting process.
The wet dechlorination relies on the property of chloride being soluble in water, hence chlorine is transferred to solution. Generally, sodium carbonate (or ammonium carbonate) is used for treating the materials, converting lead chloride and zinc chloride into carbonate which is insoluble in water, while the chlorine enters into the solution in the form of sodium chloride. The wet dechloridation has the shortcomings of incomplete dechlorination, high reagent consumption, high cost, high water consumption for the dechlorination, and the dechlorinated liquid being a mixed dilute solution of sodium (potassium) chloride, sodium carbonate and sodium (potassium) sulfate. The dechlorinated solution, which is non-recyclable and it is inconvenient to recover the chloride salts, is generally discharged. Therefore, a large amount of chlorine-containing wastewater discharge is associated with the wet dechlorination.
Despite the various disadvantages, the process of removing chlorine from the raw materials is still a process of choice to treat chlorine-containing zinc oxide secondary resources at present. After removing the chlorine from the chlorine-containing zinc oxide it can be applied to the current zinc smelting process. Due to incomplete dechlorination of the raw materials, a certain amount of chlorine is still present in the raw materials and is constantly accumulated during the leaching-electrolysis cycle in the process of zinc hydrometallurgy. Therefore, the chlorine must in turn be removed from the circulating zinc sulfate solution. The chlorine in the zinc sulfate solution is removed on the basis that some oxides have low solubility in the zinc sulfate solution and form a chloride precipitate, which allows the chlorine to be removed from the solution. Generally, monovalent copper ions serve as a precipitant for precipitating chlorine ions. By precipitating the chlorine in the form of cuprous chloride, the chlorine content in the zinc sulfate solution can be reduced so that electrolysis can be implemented normally. Ion exchange, chlorine extraction using organic solvent, or other methods are also available to transfer chlorine in the zinc sulfate solution to other solutions and be discharged in the form of wastewater. Because of existence of the chlorine, the zinc hydrometallurgy process is complex and consumption of expensive copper is increased. Meanwhile, discharge of chlorine-containing wastewater is increased by a large amount.
Therefore, the problem of high chlorine content in the secondary zinc oxide is still a challenge in the zinc recycling field. Because such a problem cannot be solved thoroughly, secondary zinc oxide resources cannot be effectively utilized for a long time. Thus, utilization rate of secondary zinc resources is much lower than that of the other non-ferrous metals.